Styli are often used as human input output devices for computers and other electronic devices and are meant to mimic the behavior of pens, pencils, brushes and other familiar writing instruments. Styli are typically used to make markings on touch screen surfaces or specially instrumented surfaces that can detect the pressure of a stylus on the surface or in other ways determine its location.
When using real pens, pencils and brushes, users will often vary pressure and angle to achieve interesting effects. A good example of this is calligraphy, where variation of the angle the nib of the pen makes with the paper creates different line widths and curves. Similarly, the angle a brush is held on a canvas determines if the brush strokes are wide or narrow. Ideally, styli should mimic this behavior of their non-electronic counterparts.